2.3.4 Volcanic and Solar Effects in the Recent Record

Recent studies comparing reconstructions of surface temperature and natural
(solar and volcanic) radiative forcing (e.g., Lean et al., 1995; Crowley and
Kim, 1996, 1999; Overpeck et al., 1997; Mann et al., 1998; Damon and Peristykh,
1999; Free and Robock, 1999; Waple et al., 2001) suggest that a combination
of solar and volcanic influences have affected large-scale temperature in past
centuries. The primary features of the Northern Hemisphere mean annual temperature
histories of Mann et al. (1999a) and Crowley and Lowery (2000) from AD 1000
to 1900 have been largely reproduced based on experiments using an Energy Balance
Model forced by estimates of these natural radiative forcings (Crowley, 2000;
Mann, 2000) making the argument that the "Little Ice Age" and "Medieval
Warm Period", at the hemispheric mean scale, are consistent with estimates
of naturally-forced climate variability. Several studies indicate that the combined
effect of these influences has contributed a small component to the warming
of the 20th century. Most of these studies isolate greenhouse radiative forcing
as being dominant during late 20th century warming (see Crowley, 2000). This
argues against a close empirical relationship between certain sun-climate parameters
and large-scale temperature that has been claimed for the 20th century (Hoyt
and Schatten, 1997). The reader is referred to Chapter 6
for a detailed discussion of these radiative forcings, and to Chapter
12 for comparisons of observed and model simulations of recent climate change.

2.3.5 Summary

Since the SAR there have been considerable advances in our knowledge of temperature
change over the last millennium. It is likely that temperatures were relatively
warm in the Northern Hemisphere as a whole during the earlier centuries of the
millennium, but it is much less likely that a globally-synchronous, well defined
interval of "Medieval warmth" existed, comparable to the near global
warmth of the late 20th century. Marked warmth seems to have been confined to
Europe and regions neighbouring the North Atlantic. Relatively colder hemispheric
or global-scale conditions did appear to set in after about AD 1400 and persist
through the 19th century, but peak coldness is observed during substantially
different epochs in different regions. By contrast, the warming of the 20th
century has had a much more convincing global signature (see Figure
2.9). This is consistent with the palaeoclimate evidence that the rate and
magnitude of global or hemispheric surface 20th century warming is likely to
have been the largest of the millennium, with the 1990s and 1998 likely to have
been the warmest decade and year, respectively, in the Northern Hemisphere.
Independent estimates of hemispheric and global ground temperature trends over
the past five centuries from sub-surface information contained in borehole data
confirm the conclusion that late 20th century warmth is anomalous in a long-term
context. Decreasing temporal resolution back in time of these estimates and
potential complications in inferring surface air temperature trends from sub-surface
ground temperature measurements precludes, however, a meaningful direct comparison
of the borehole estimates with high-resolution temperature estimates based on
other proxy climate data. Because less data are available, less is known about
annual averages prior to 1,000 years before the present and for conditions prevailing
in most of the Southern Hemisphere prior to 1861.